A. Field of the Invention
The present invention relates to a graft copolymer of polyamide and a glycidyl group-containing acrylate copolymer and a process for preparing the graft copolymer. The present invention further relates to a coating composition containing the graft copolymer, particularly a powder coating composition.
B. Description of the Related Art
Over the years, society has greatly benefitted from the development of a wide spectrum of polymeric materials. Polymers have been used in almost every phase of everyday life and have found particular applicability in automotive parts, containers, fibers, filaments, fabric, construction materials, adhesives and coatings. Due to the diverse characteristics of different polymers, some polymers are especially useful in certain environments while others are contraindicated. In an effort to alleviate the less advantageous properties of polymers, attempts have been made to modify polymers by blending additives or even other polymers. In addition, attempts have been made to chemically modify polymers by adding reactive groups, by copolymerization with compatible monomers, by block copolymerization or by graft copolymerization.
In the field of coatings, particularly powdered coatings, polymeric materials have played a prominent role. For instance, U.S. Pat. No. 4,042,645, reissued as Reissue Pat. No. 32,261, describes a thermosetting powder coating composition obtained by mixing a major proportion of a solid copolymer prepared from defined amounts of (A) a (meth)acrylate ester, (B) an xcex1,xcex2-ethylenically unsaturated carboxylic acid or anhydride or a glycidyl acrylate or methacrylate, optionally (C) a monomer copolymerizable with (A) and (B), and a minor portion of a cross-linking compound or an epoxy resin containing at least two epoxy radicals in the molecule in the presence of a tertiary amine curing accelerator.
To modify the properties of the powder coatings, blends with additives or other polymers have typically been employed. However, one of the drawbacks of blends is that a hazy or opaque coating often is obtained. While such a result might be acceptable if the coating is to be pigmented, it is unacceptable if the haze or opacity adversely affects the aesthetics of the coating or if a clear coat is desired.
Further illustrative of the art relating to powder coatings is U.S. Pat. No. 5,407,706 which describes a powder coating composition that provides low gloss upon curing. The composition comprises (A) a resin comprising from 10 to 90 weight % of an acrylic resin having a viscosity of 100 to 800 poises at 140xc2x0 C. that is obtained by polymerizing 10 to 50 weight % of glycidyl acrylate or glycidyl methacrylate with 90 to 50 weight % of a copolymerizable monomer and 90 to 10 weight % of a further acrylic resin having a viscosity of 1,000 to 5,000 poises at 140xc2x0 C. that is prepared from defined comonomers, and (B) a polybasic acid compound having a viscosity of 100 to 2,000 poises at 150xc2x0 C. The equivalent ratio of the glycidyl groups to the acid groups of the polybasic acid compound may be from 1.5 to 0.5.
U.S. Pat. No. 5,436,311 describes a powder thermosetting composition comprising as binder a mixture of a linear carboxyl group-containing polyester and a glycidyl group containing acrylic copolymer. The polyester has an acid number of 20 to 50 mg KOH/g. The acrylic copolymer has a number average molecular weight of from 4,000 to 10,000 and is obtained from 5 to 30% by weight glycidyl acrylate or glycidyl methacrylate and 70 to 95% by weight of methyl methacrylate whereby up to 25% by weight of the methyl methacrylate can be replaced by another vinyl monomer.
U.S. Pat. No. 5,744,522 relates to a low gloss coating composition containing a glycidyl group-containing acrylic copolymer, an aromatic polyester and a defined isocyanurate curing agent. The background of this patent provides a description of the previously described patents and other documents relating to coating compositions.
In addition to various acrylic polymers, the art has developed certain modified polyamides. Illustrative of such modified polyamides is U.S. Pat. No. 4,973,617 which relates to a water-borne printing ink composition based on acrylic resins and maleated rosin modified polyamides. The modified polyamides are said to provide good adhesion, clean printing, excellent film wetting and superior resolubility and the resulting ink compositions are said to be especially useful for printing onto plastic substrates.
U.S. Pat. No. 5,574,101 describes an acrylic resin composition comprising at least one polyamide elastomer consisting of hard segments and soft segments, an acrylic resin and optionally at least one electrolyte. The composition is said to possess permanent anti-static properties and good transparency which is only slightly deteriorated even when immersed in water. The compositions are disclosed as being useful for parts of electronic products, household appliances, office automation appliances and other devices.
Japanese Unexamined Patent Publication No. 02-060930 relates to graft copolymers which are said to exhibit transparency, flexibility and heat resistance which are useful for the preparation of coatings, adhesives, etc. The graft copolymers are prepared by polycondensing a polyalkyl (meth)acrylate, such as polybutyl acrylate, a dicarboxylic acid and an aromatic diamine.
In one aspect, the present invention provides a graft copolymer comprising a polyamide to which is grafted a glycidyl group-containing acrylate copolymer.
In a further aspect, the present invention provides a process for preparing a graft copolymer comprising a polyamide to which is grafted a glycidyl group-containing acrylate copolymer. The process comprises:
A) dispersing in an organic solvent a polyamide and a material which will react with the polyamide to form the graft copolymer of the polyamide and the glycidyl group-containing acrylate copolymer; and
B) reacting the polyamide and the material so as to form said graft copolymer.
In a still further aspect, the present invention provides coating compositions comprised of the graft copolymer.
As noted above, one aspect of the present invention relates to a graft copolymer comprising a polyamide to which is grafted a glycidyl group-containing acrylate copolymer. The polyamide preferably exhibits a relatively low melting point which is less than about 160xc2x0 C., more preferably less than about 145xc2x0 C., as determined by a differential scanning calorimeter and ASTM B3418. Polyamides of this type generally have a weight average molecular weight greater than about 100,000 as determined by gel permeation chromatography.
Polyamides which can be used in the present invention are known in the art. For instance, one type of polyamide which can be used is a block copolymer prepared from nylon 12 (polydodecanolactam) and polytetramethylene ether glycol. Such polyamide is commercially available from Elf Atochem under the designations Pebax 2533, 3533 and 5533.
The glycidyl group-containing acrylate copolymer can be grafted to the polyamide in a process which involves sufficiently dispersing the polyamide in an organic solvent selected so that the polyamide can be reacted to form the graft copolymer. The organic solvent is typically a non-polar aromatic solvent such as xylene, toluene or commercially available proprietary solvents, such as Aromatic 100, or mixtures thereof and is preferably selected so that the polyamide can be totally dispersed (i.e., dissolved) in the solvent. The preferred organic solvent is xylene. Although not critical, the polyamide is dispersed in an amount ranging from about 2 to about 25% by weight of the organic solvent.
The material which forms the graft copolymer with the polyamide can be added with the polyamide into the organic solvent, but is preferably added after the polyamide has been dispersed. As used in the present invention, the term xe2x80x9cmaterialxe2x80x9d means a component or plurality of components that can react with the polyamide to form the graft copolymer of the polyamide and the glycidyl group-containing acrylate copolymer. More specifically, the material can be an unpolymerized mixture of ethylenically unsaturated monomers and initiator or a preformed polymer solution containing the initiator. The mixture of monomers comprises at least one compound of formula (I) and at least one ethylenically unsaturated compound that is copolymerizable with the monomer(s) of formula (I). Formula (I) is set forth as follows: 
wherein R1 represents H or an alkyl group containing from 1 to 4 carbon atoms, R2 represents a branched or unbranched alkyl group containing from 1 to 20 carbon atoms, and R3 represents H or an alkyl group containing from 1 to 4 carbon atoms. Illustrative compounds within the definition of formula (I) are glycidyl acrylate, glycidylmethacrylate, 1,2-epoxybutylacrylate and betamethyl glycidyl methacrylate. Glycidyl methacrylate is the preferred compound of formula (I).
The compound of formula (I) generally is present in amount of from about 10 to about 70%, preferably from about 16 to about 50% by weight of the precursors forming the glycidyl group-containing acrylate copolymer.
The ethylenically unsaturated compound copolymerizable with the compound of formula (I) can include alkyl esters of acrylic acid or methacrylic acid such as methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate, isobornylacrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, 2-ethylhexylmethacrylate, lauryl methacrylate, tridecyl methacrylate, stearyl methacrylate, cyclohexyl methacrylate, isobornyl methacrylate and mixtures thereof. Preferred alkyl esters of acrylic acid or methacrylic acid are methyl methacrylate and n-butyl methacrylate and especially preferred is a mixture of methyl methacrylate and n-butyl methacrylate.
The ethylenically unsaturated compound can further include vinyl monomers such as styrene, vinyltoluene, xcex1-methylstyrene; acrylonitriles, for example, acrylonitrile and methacrylonitrile; acrylamides, for example, acrylamide and dimethylacrylamide; hydroxyalkyl esters of acrylic acid and methacrylic acid, for example, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, and hydroxypropyl methacrylate; and dialkyl esters of unsaturated dibasic acids. The ethylenically unsaturated compound copolymerizable with the compound of formula (I) can also be mixtures of the aforementioned compounds. Other components may be present as long as they do not substantially adversely affect the results of the invention. It is preferred that the ethylenically unsaturated compound includes a vinyl monomer with styrene being especially preferred. A preferred material is a mixture of styrene, methyl methacrylate and n-butyl methacrylate which is reacted with a compound of formula (I), especially glycidyl methacrylate, to form a glycidyl group-containing copolymer that is grafted to the polyamide.
Although monomers are typically used so as to form a glycidyl group-containing acrylate copolymer and the graft copolymer in the same reaction medium, it is possible that higher molecular weight entities (e.g., oligomers or polymers) can be used as long as such entities can form the graft copolymer of polyamide and glycidyl group-containing acrylate copolymer. Thus, for instance, it is possible to form the graft copolymer by dispersing the polyamide and a polymer previously formed from at least one compound of formula (I) and at least one ethylenically unsaturated compound copolymerizable with the compound of formula (I) and reacting the polyamide and the pre-formed glycidyl group-containing acrylate copolymer to form the graft copolymer.
The ethylenically unsaturated compound copolymerizable with the compound of formula (I) is generally present in amount of from about 30 to about 90%, preferably from about 50 to about 84% by weight of the components forming the glycidyl group-containing acrylate copolymer.
The amounts of the polyamide and the material which forms the graft copolymer with the polyamide are generally selected so that the graft copolymer is comprised of from about 2 to about 50% by weight, preferably from about 5 to about 20% by weight of the polyamide and from about 50 to about 98% by weight, preferably from about 80 to about 95% by weight of the glycidyl group-containing acrylate copolymer.
To the dispersion containing the polyamide and the material which forms the graft copolymer with the polyamide is typically added an initiator for the graft copolymerization reaction. An acceptable initiator is one that generates free radicals that are sufficiently energetic to abstract hydrogen atoms from the polyamide during the reaction. One such group of initiators is those compounds which can form methyl radicals either directly or by decomposition of the primary radical to a methyl radical. Illustrative of this type of initiator which can be used in the present invention are t-butyl peroctoate, t-butylperoxyacetate, di-t-butylperoxide and mixtures thereof. The initiator can be present in an amount of from about 1 to about 12% by weight of the material used to prepare the graft copolymer with the polyamide.
It is also possible to use combinations of initiators which can provide a desired degree of grafting. For instance, the previously described type of initiator can be mixed with one or more initiators that are not sufficiently energetic to abstract hydrogen atoms. One example of this type of initiator is di-t-amyl peroxide. Thus, by selecting combinations of initiators in a manner known in the art, one can modify the characteristics of the graft copolymer and thus the final coating with respect to characteristics such as chip resistance and impact resistance.
The dispersion containing the polyamide and the material which forms the graft copolymer with the polyamide can also include a chain transfer agent, such as n-dodecyl mercaptan, t-dodecylmercaptan and mixtures thereof, in an amount of from about 0.1 to about 10% by weight of the polyamide and material used to prepare the graft copolymer. Other materials that may also be present are conventional: additive, such as thermal stabilizers, in amounts that are conventional in the art. One unexpected advantage of the graft copolymers of the present invention is that they can provide superior flow control even though they can exhibit high melt viscosity.
The reaction between the polyamide and the material which forms the graft copolymer with the polyamide is typically conducted under an inert atmosphere, such as nitrogen, at an elevated pressure of from about 1 to about 50 psig. The reaction is generally conducted within the temperature range of from about 90 to about 160xc2x0 C. for a time ranging from about 1 to about 10 hours. During the polymerization reaction, the dispersion is stirred. Upon completion of the acrylic polymerization, the graft copolymer of the polyamide and the glycidyl group-containing acrylate copolymer is typically recovered by stripping the organic solvent from the graft copolymer under an elevated temperature and reduced pressure in a manner known in the art.
While not being limited to any particular theory, it is believed that active site(s) on the polyamide (i.e., free radicals) initiate an acrylic polymerization or terminate a growing acrylic polymer chain thereby forming the graft copolymer. It is possible that the polyamide can have multiple bonds to the glycidyl group-containing copolymer. It is further possible that some of the bonds with the polyamide can form via reaction of the epoxy group of the glycidyl group-containing acrylate copolymer with the NH-groups of the polyamide.
Although it is believed that all the polyamide molecules are reacted with the glycidyl group-containing acrylate copolymer, not all the glycidyl group-containing acrylate copolymer is reacted with nylon. This is particularly the case where an unpolymerized mixture of monomers comprising at least one compound of formula (I) and at least one ethylenically unsaturated compound that is copolymerizable with the monomer(s) of formula (I) is used with the polyamide so as to form the graft copolymer. The mixture of the graft copolymer and the glycidyl group-containing acrylate copolymer resulting from the reaction can be used as is or can be subjected to conventional separation techniques in order to recover the graft copolymer. The separated graft copolymer can then be used as an additive to modify the characteristics of conventional coating materials, particularly acrylic coating materials.
The graft copolymer typically has a weight average molecular weight of greater than about 120,000 as determined by gel permeation chromatography relative to a polystyrene standard. Where a mixture of the graft copolymer and the glycidyl group-containing acrylate copolymer is obtained and the graft copolymer is not separated from the glycidyl group-containing acrylate copolymer, the weight average molecular weight exhibits a bimodal molecular weight distribution with one peak on the order of about 4,000 to 12,000 which indicates the glycidyl group-containing acrylate copolymer and a further peak on the order of about 80,000 to 120,000 (or greater) which indicates the graft copolymer.
The graft copolymer of the polyamide and the glycidyl group-containing acrylate copolymer has a high degree of optical transparency compared with blends of the polyamide and the glycidyl group-containing acrylate copolymer.
The graft copolymer of the present invention can be used to prepare a variety of materials including molded articles. One advantage of the graft copolymer in prepared molded articles is the substantial reduction in bubbles, such as caused by air entrapment, despite a rapid increase in viscosity upon cooling.
The graft copolymer of the present invention is particularly useful in coating compositions. The coating composition can contain the graft copolymer in a solution or dispersion of an organic solvent, but it is especially preferred to prepare a coating composition which contains the graft copolymer in the form of a powder. Other components of the coating composition can include a polyester which is present in an amount ranging from about 2 to about 25%, preferably from about 5 to about 15% by weight of the graft copolymer. A particularly suitable polyester for use with the graft copolymer of the present invention is a commercially available aliphatic polyester having an acid number of approximately 54-58, a hydroxyl number less than 3.0, a maximum APHA color of 50, a specific gravity at 25xc2x0 C. of 1.162, a melting point (as determined by a differential scanning calorimeter) of approximately 100xc2x0 C. and a melt viscosity of approximately 2400-2700 cps at 100xc2x0 C. (as determined by an ICI Cone and Plate Viscometer).
In addition, the coating composition can contain curing agents, such as 1,12-dodecanedioic acid (e.g., available from E.I. Dupont de Nemours and Co., Inc.) and 1,3,4-butanetricarboxylic acid (e.g., available from Mitsubishi Chemicals Inc.), the amount of which is based on epoxy equivalent weight and the desired properties of the cured coating. The coating composition can further contain conventional additives, such as ultraviolet absorbers (e.g., Tinuvin 900 from Ciba Geigy Corp.), hindered amine light stabilizers (e.g., Tinuvin 144 from Ciba Geigy Corp.), volatile release agents, such as benzoin (e.g., Uraflow B from GCA Chemical Co. of Bradenton, Fla.), in amounts known in the art. While the graft copolymers exhibit improved flow characteristics which can obviate the need of a flow modifier, such a modifier, e.g., Modaflow Powder III (polyacrylate flow modifier from Monsanto Co.), can also be added. The coating composition can be used to provide a clear coat, but fillers and pigments can also be incorporated into the coating composition in known amounts as the situation warrants.
A powdered coating composition can be prepared by mixing the graft copolymer comprised of the polyamide and the glycidyl group-containing acrylate copolymer and the other components of the composition, extruding the mixture, grinding the mixture into the form of powder that can be used in conventional powder coating apparatus, such as powder exhibiting an average particle size of from about 10 to about 70 microns. A convenient way of obtaining powder of appropriate size is by sieving, such as by using powder that passes through a 140 or 170 mesh screen.
A specific illustration of the foregoing technique on a small scale is to use a Vitamixer blender of the Vitamix Corporation in Cleveland, Ohio to form the initial mixture of the graft copolymer and the other components of the coating composition. The mixed components are then placed in a heated extruder where the mixture is melt mixed and extruded, such as an APV Model 19 PC twin screw extruder with two individually adjustable heating zones with a variable rotation rate that can provide an extrudate in ribbon form from between a pair of chilled pinch rolls. The extruded composition is then crushed into powder form by any suitable means, such as a hammer mill (or a Vitamixer blender for small quantities) and powder passing through a 140 or 170 mesh sieve is collected. The powder can be packaged and used for coating various articles.
The advantages of toughness and reduced friability that can be attained with the graft copolymer of the present invention is particularly manifested in the preparation of a powdered coating composition. The improved toughness of the graft copolymer is demonstrated by increased grinding times relative to conventional polymeric powder coating compositions in order to obtain the desired particle size. However, despite the longer grinding times, the amount of fines is substantially reduced thereby illustrating the improved friability. Since much of the fines are typically removed by particle classifiers (e.g., particle sieves) and discarded, it can be understood that the reduced friability that can be obtained from the present invention can result in improved process efficiency while the improved toughness can provide improved impact resistance and chip resistance that are important factors for coating compositions, particularly in the automotive coating area.
To apply the coating composition to a surface, conventional techniques can be used so as to obtain a smooth, substantially uniform coating. For instance, when the coating is to be applied to conductive substrates, such as steel articles (which have typically been pre-treated with iron or zinc phosphate), electrostatic spraying can be used. Spraying equipment is commercially available from manufacturers such as GEMA Volstatic of Indianapolis, Ind. and The Nordson Corp. of Amherst, Ohio.
Curing is achieved by heating the coated surface for a time sufficient to cure the composition. Although the specific curing conditions depend on the precise constituents of the composition, including the presence or absence of a cure catalyst, typical cure conditions without the presence of a catalyst are from about 20 to about 30 minutes at about 160 to about 195xc2x0 C. As an illustration, typical curing conditions for a cured coating of 3 mils (approx. 80 microns) is 30 minutes at about 165xc2x0 C. In the presence of conventional cure catalysts, such as dimethylcocoamine, in amounts known in the art, cure temperatures can be as low as 135xc2x0 C.
By following the teachings of the present invention, the cured composition can provide a smooth glossy finish that exhibits a brightness characteristic of high refractive index compositions. Unlike blends of the polyamide and the glycidyl group-containing acrylate copolymer which are cloudy or opaque, the graft copolymers can provide optical transparency which is vital in the preparation of clearcoat compositions. The smooth glossy finish that can be obtained in accordance with the present invention is especially surprising in view of the high melt viscosity that is exhibited by some of the graft copolymers. Less smoothness and lower gloss would normally be expected from materials exhibiting a higher melt viscosity. Therefore, it can be understood that the graft copolymers exhibit superior flow properties. This characteristic enables thinner powder coatings to be used to attain the same appearance comparable to liquid coatings. If one can use thinner powder coatings while obtaining the same surface appearance as thicker coatings made from conventional powder material, significant savings in material and an improvement in process efficiency can be attained. In addition, if a flow control agent can be reduced in amount or eliminated entirely, the gloss can be improved since a flow control agent typically concentrates on the surface of the cured coating.
As indicated above, the cured compositions have reduced friability and brittleness compared to conventional compositions prepared from glycidyl methacrylate polymers which can provide improved impact resistance and chip resistance as determined by gravelometer tests. Yet, the compositions prior to curing can be ground and sieved with conventional apparatus to provide a powder coating material that can be applied with conventional powder coating devices and cured under conventional conditions.